Enhanced Visible Light Photocatalysis of Bi2O3 upon Fluorination

Sep 9, 2013 - (1-3) However, its requirement of UV-irradiation activation seriously hinders .... Figure 1a displays the XRD patterns of the pure Bi2O3...
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Enhanced Visible Light Photocatalysis of Bi2O3 upon Fluorination Hai-Ying Jiang, Jingjing Liu, Kun Cheng, Wenbin Sun, and Jun Lin* Department of Chemistry, Renmin University of China, Beijing 100872, People’s Republic of China S Supporting Information *

ABSTRACT: For the better utilization of solar light and complete oxidation of environmental organic pollutants, it is desired to develop small band gap semiconductors with a deep valence band as efficient visible light photocatalysts. In this work, we prepared the fluorinated Bi2O3 catalysts using a precipitation method, followed by a solvothermal process in the presence of NH4F. The fluorinated Bi2O3 catalysts, especially with the atomic ratio of F to Bi (RF) at 0.2, exhibit much higher photocatalytic activities than the pure Bi2O3 for the degradation of methyl orange (MO) under the visible light irradiation. The effects of the fluorination on the phase structure, special surface areas, morphologies, optical properties, surface-adsorbed species, and electronic band structure of the Bi2O3 were investigated in detail. It was revealed that both the surface-adsorbed and lattice-substituted fluorine, induced by the fluorination to Bi2O3, play critical roles in the enhanced photocatalytic performance of the fluorinated Bi2O3. The two types of fluorine species effectively inhibit the recombination of the photoexcited electron−hole pairs by withdrawing the photoexcited electrons and increase the oxidation power of the photoexcited hole by lowering the valence band edge, respectively. TiO2 was significantly enhanced by surface fluorine modification or fluorine doping.20−25 In addition, the improvement of photocatalytic activity was also observed over other fluorinated semiconductors such as SrTiO3, ZnWO4, and Bi2WO6.26−28 According to these examples, we wonder if the fluorination would be also an effective way to enhance the photocatalytic performance of Bi2O3. To our best knowledge, as of now, the fluorinated Bi2O3 as a photocatalytic material has not been reported yet. Aiming at developing Bi2O3 as an efficient visible light photocatalyst, we prepared the fluorinated Bi2O3 with different atomic ratios of fluorine to bismuth by a facile approach in the present work. Upon visible light irradiation, the photocatalytic activity of Bi2O3 for the degradation of methyl orange (MO) has been shown to be remarkably enhanced after the fluorination. Furthermore, the role of the fluorination in the enhanced photocatalytic activity was revealed based on the results of various physicochemical characterizations and the analysis of the radical species formed in the visible light irradiated methanol suspension of pure Bi2O3 and fluorinated Bi 2 O 3 . This work evidences that the fluorination of semiconductor photocatalysts is a versatile method for improving the photocatalytic efficiencies of Bi2O3 and would be helpful for the development of the small band gap semiconductors with a deep valence band as efficient photocatalytic materials.

1. INTRODUCTION Semiconductor photocatalysis with a primary focus on anatase TiO2 has been extensively investigated as an efficient method for environmental pollution remediation and solar energy conversion.1−3 However, its requirement of UV-irradiation activation seriously hinders the transition from basic research toward practical application. Therefore, many strategies have been proposed to develop visible light photocatalystic materials for the better utilization of solar light.4−7 One noteworthy strategy is the development of the narrow band gap semiconductors as visible light photocatalystic materials.8−11 Among the various narrow band gap semiconductors, bismuth oxide (Bi2O3) with band gap varying from 2.1 to 2.8 eV is a prospective candidate because of its unique characteristics including adequately high oxidation power of valence hole (∼+3.13 V vs NHE) and nontoxic property as TiO2.12−14 In the viewpoints of the efficient utilization of solar light, complete oxidation of environmental pollutants, and environmental friendly features, it is desired and meaningful to develop bismuth oxide (Bi2O3) as an efficient visible light photocatalytic material. However, pure Bi2O3 as a photocatalyst was found not to be as good as expected since its inability of the conduction band (CB) electron (∼+0.33 V vs NHE) to scavenge surface oxygen molecule (E°(O2/O2•−) = −0.33 V vs NHE) results in a fast recombination of the photoexcited electron and hole. To solve this problem, a lot of works have been carried out, including the microstructure control and the surface deposition of noble metal.15−18 It was widely reported that the charge transfer against recombination can be effectively achieved by the fluorination of semiconductor photocatalysts.19 Numerous works clearly demonstrated that the photocatalytic activity of semiconductor © 2013 American Chemical Society

Received: July 11, 2013 Revised: August 17, 2013 Published: September 9, 2013 20029

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2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All chemicals used in this study were analytic grade reagents without further purification. Fluorinated Bi2O3 samples were prepared using a precipitation method, followed by a solvothermal process. In detail, 10.78 g of Bi(NO3)3·5H2O was dissolved in a 30 mL aqueous solution of HNO3 (1.5 M). Under vigorous agitation, into the solution was NaOH solution (50% w/v) added dropwise until pH = 13, at which a yellow precipitate formed. Subsequently, the mixture was heated at 80 °C for 2 h, and then the resulting precipitate was collected by centrifugation and washed with deionized water and ethanol for several times before being dried at 100 °C for 12 h. Afterward, the freshly obtained precipitate (2.517 g) above was dispersed into a 20 mL mixed solution of ethanol−water (v/v = 9:1) containing the desired amount of NH4F·H2O. The atomic ratios of F to Bi (RF) in the dispersions are 0.0, 0.1, 0.2, and 0.3, respectively. The dispersion was then transferred to a Teflon-lined autoclave and treated at 150 °C for 4 h. After the reaction was completed, the yellow precipitate was recovered by centrifugation and washed with deionized water several times. Finally, the samples were calcined at 300 °C for 2 h to form the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values. 2.2. Characterization. X-ray diffraction (XRD) patterns of the as-prepared samples were recorded on an X-ray diffractometer (Shimadzu, XRD-7000) using Cu Kα as X-ray radiation (λ = 0.154 18 nm). The accelerating voltage and applied current are 40 kV and 30 mA, respectively. Fourier transform infrared (FTIR) spectra were obtained with an IRPrestige-21 FTIR spectrophotometer. The morphologies of these samples were observed by using a field-emission scanning electron microscope (FESEM) (JEOL JSM-7401E). The Brunauer−Emmett−Teller (BET) special surface areas were determined through nitrogen adsorption at 77 K using a Micromeritics Tristar 3020II instrument. X-ray photoelectron spectroscopy (XPS) and valence band X-ray photoelectron spectroscopy (VB XPS) were performed on an ESCALAB 250Xi spectrometer equipped with 300 W Al Kα radiation. All binding energies were referenced to the C 1s peak (284.6 eV) of the surface adventitious carbon. The optical absorbance spectra of the samples were recorded on a UV−vis spectrophotometer (Hitachi U-3900) equipped with a diffuse reflectance accessory, and BaSO4 was used as the reflectance standard. 2.3. Photocatalytic Activity Experiments. The degradation of methyl orange (MO) was carried out to evaluate the photocatalytic activities of the as-prepared pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values. Typically, 10 mg sample was suspended in 100 mL aqueous solution of MO (4 × 10−5 M). The light source was a 300 W Xe arc lamp (CHFXM150, Beijing Trusttech. Co. Ltd.) equipped with a wavelength cutoff filter for λ > 420 nm and positioned about 8 cm above the aqueous suspension. Prior to irradiation, the suspension was magnetically stirred in the dark for more than an hour to ensure the establishment of the equilibrium between the catalyst surface and MO molecules. At the given irradiation time intervals, 3 mL of the suspension was sampled, and then the catalyst and the MO solution were separated by centrifugation. The concentration of MO was determined by monitoring the change of the absorption spectrum in the absorbance at 461 nm with a Hitachi U-3310 spectrophotometer.

To study the photocatalytic oxidation pathway over these catalysts, triethanolamine (TEOA, 10 mM) as an effective hole scavenger and tert-butyl alcohol (TBA, 10 mM) as •OH radicals scavenger (with a rate constant k = 6 × 108) were chosen, respectively, to participate in the photocatalytic degradation of MO over these catalysts.29 The determination of MO concentration during the photocatalytic reaction in the presence of TEOA or TBA was also conducted by measuring the absorption of MO solution at 461 nm. 2.4. ESR Measurement. To investigate the separation degree of the photoexcited electron−hole pairs over the asprepared catalysts, the spin trap method was employed using diamagnetic DMPO to produce the stable paramagnetic spinadduct with •CH2OH radicals formed in the visible light irradiated methanol suspensions of the catalysts. Prior to ESR measurement, the methanol suspension of the catalyst was bubbled with Ar gas for several minutes to remove the soluble oxygen. The DMPO−•CH2OH spin adduct was detected by an electron spin resonance spectrometer (ESR) (JEOL JES-FA 200) equipped with 500 Xe lamp as the irradiation source (λ > 420 nm). The settings for ESR spectrometer were centerfield = 323.17 mT, sweep width = 5 mT, microwave frequency = 9056 MHz, and microwave power = 7.99 mW.

3. RESULTS AND DISCUSSION 3.1. Characterizations. XRD was used to indentify the phase structures and purities of the as-prepared samples. Figure 1a displays the XRD patterns of the pure Bi2O3 (RF = 0) and

Figure 1. (a) X-ray diffraction patterns of pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values. (b) Diffraction peak positions of the crystal plane (120) in the range of 2θ = 26.9°−27.6°. 20030

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fluorinated Bi2O3 with different RF values. It is clearly observed that all samples exhibit a single monoclinic phase of wellcrystalline α-Bi2O3 (JCPDS No. 41-1449) with an exception of the sample RF = 0.3, in which minor impurity’s diffraction peaks are found. The minor impurity could be indentified to be BiOF according to JCPDS file (No. 73-1595). Further observation indicates that with the increase in RF value the XRD peak intensities of Bi2O3 phase slightly increase, and the widths of the XRD peaks appear to be sharper, indicating that the crystallization and crystal growth of the Bi2O3 are slightly enhanced. A similar phenomenon was also reported to occur on the fluorinated TiO2,21,30 probably caused by the mediating role of fluoride ions in the dissolution and recrystallization processes of metastable metal oxide intermediates.19,31 A comparison of the (120) diffraction peaks of the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values in the range of 2θ = 26.9°−27.6° (see Figure 1b) shows that the (120) peak position obviously shifts toward a higher 2θ value with the increase of RF value. Based on Bragg’s law, the observed shift of the diffraction peaks to a higher angle directly reflects the lattice shrinkage of the detected crystals. The lattice shrinkage well suggests the substitution of the foreign F− anion with smaller size (0.133 nm) for the host O2− anion with larger size (0.14 nm) in the Bi2O3 host after the fluorination. Besides, the substitution of F− anions for O2− anions should cause the generation of Bi vacancies in the Bi2O3 host for charge compensation, which also contributes to the lattice shrinkage. The XRD results reveal that the fluorination on Bi2O3 in the present case not only introduces fluorine as substituting anion into the Bi2O3 host but also slightly improves the crystallinity of the Bi2O3. Figure 2 shows the FT-IR spectra of the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values. It can be

peak at 518 cm−1 shifts to a higher wavenumber (530 cm−1), accompanying an emergence of a new peak occurring at 495 cm−1. Both the peaks show a trend of increase in strength with the increase of the RF value. According to the related literature,33 the peak at 495 cm−1 can be assigned to the stretching vibrations of Bi−F bonds. The location shift of the Bi−O bond stretching vibrations in the spectra of the fluorinated Bi2O3 probably originates from the change in the coordination environment surrounding Bi after the substitution of F− for O2− in the Bi2O3 host. The BET surface areas of the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values are presented in Table 1. It can be seen that the surface area of the Bi2O3 is

Figure 2. (a) FT-IR spectra of pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values. (b) Enlarged FT-IR spectra in the region of 600−400 cm−1.

Figure 3. UV−vis diffuse reflectance spectra of pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values. The inset is the plot used to estimate the band gap value.

observed in Figure 2a that as compared to the fluorinated Bi2O3 samples, the pure Bi2O3 exhibits a broader band around 3450 cm−1, which is associated with the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups. The enlarged FT-IR spectra in the region of 600−400 cm−1 is given in Figure 2b to clearly indentify the characteristic modes in these samples. In the spectrum of the pure Bi2O3 (RF = 0), two peaks located at 518 and 430 cm−1 are ascribed to the stretching vibrations of Bi−O bonds of BiO6 coordination polyhedron in α-Bi2O3 crystals.32 After the fluorination, the

be seen that the fluorination obviously affects the light absorption characteristics of the Bi2O3. The pure Bi2O3 (RF = 0) exhibits an intense absorption with the threshold at approximately 455 nm, which corresponds to the intrinsic band gap absorption of α-Bi2O3. The fluorination of the Bi2O3 leads to an obvious blue-shift in the absorption threshold, indicating an increase in the band gap. In the plots of (αhv)1/2 vs photo energy as shown in the inset of Figure 3, the band gaps of the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF =

Table 1. BET Surface Areas of Fluorinated Bi2O3 with Different RF Values

a

RFa

BET surf. area (m2/g)

RFa

BET surf. area (m2/g)

0 0.1

0.392 0.802

0.2 0.3

0.659 0.692

Atomic ratios of F to Bi.

apparently increased upon the fluorination, elucidating that the fluorination can largely inhibit the interconnection of sample particles. The FESEM observation (see Figure S1 in the Supporting Information) indicates that the fluorination on Bi2O3 (RF = 0.2) has no obvious influences on the morphology and size. Both morphologies before and after fluorination appear to be microrod-shaped, which is similar to that of the αBi2O3 previously reported.17,34 The optical properties of the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values are revealed by the UV−vis diffuse reflectance spectra, as shown in Figure 3. It can

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The high-resolution XPS spectrum of the F 1s region for the fluorinated Bi2O3 (RF = 0.2) is given in Figure 5. As revealed in

0.2) are estimated from the tangent lines to be 2.68 and 2.74 eV, respectively. To further explore the chemical states of the various elements in these samples, the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2) were characterized by XPS analysis. Figure 4a presents the Bi 4f XPS spectra of the two

Figure 5. High-resolution XPS spectrum of the F 1s region for the fluorinated Bi2O3 (RF = 0.2).

Figure 5, there are two types of fluorine species in the fluorinated Bi2O3 (RF = 0.2). The peak at the binding energy of 683.6 eV can be assigned to the F species adsorbed on the host surface, while the peak at 685.7 eV is the F species substituting for O2− in the host lattice.37 Apparently, the F content on the host surface is much more than that in the host lattice. The existence of the surface and lattice fluorine species is well consistent with the XPS measurement results of the Bi 4f and O 1s species in the fluorinated Bi2O3 above. The XPS measurements were also applied to determine the total density of states (DOS) distribution of the valence band (VB) for the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2), as shown in Figure 6. It can be clearly seen that a slight

Figure 4. Bi 4f (a) and O 1s (b) XPS spectra of pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2).

samples. It can be observed that two peaks for the Bi 4f of the pure Bi2O3 are located at the binding energies of 158.9 and 164.2 eV, ascribed to Bi 4f7/2 and Bi 4f5/2, respectively, which is a characteristic of Bi3+ in α-Bi2O3.17,35 However, the binding energies of Bi 4f7/2 and Bi 4f5/2 shift to 159.1 and 164.4 eV, respectively, after the fluorination in the case of the fluorinated Bi2O3 (RF = 0.2). This observed shift to the higher binding energies clearly indicates the change in the chemical environment surrounding Bi and supports the replacement of O2− by F− with greater electronegativity, forming the mixed states such as F−Bi−O in the host lattice. The corresponding O 1s XPS profiles (Figure 4b) show that there are two surface oxygen species observed in both samples. The binding energy at the range of 529−530 eV is a characteristic of the lattice oxygen (OL), whereas the binding energy of 531−532 eV could be attributed to the surface oxygen (OS) present as hydroxyl group (−OH) bonded to surface.36 On the basis of the relative XPS area, the atomic ratio of the surface oxygen (OS) to the total oxygen (OS + OL) is dramatically decreased from 41% to 29% after the fluorination (RF = 0.2). The decrease in the amount of the surface oxygen (OS) would be due to the replacement of considerable surface groups (−OH) by−F species after the fluorination.

Figure 6. Valence band XPS spectra of pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2).

shift in the relative position of the valence band maximum (VBM) occurs after the fluorination. The VBM of the fluorinated Bi2O3 (RF = 0.2) is approximately 0.06 eV more positive than that of the pure Bi2O3 (RF = 0). The shift value is well comparable to the observed increase of the band gap from the diffuse reflectance spectra measurements (Figure 3), indicating the increase in the band gap of the fluorinated Bi2O3 (RF = 0.2) originates from the lowering of its VBM. The VBM of Bi2O3 is composed of the hybrid of O 2p and Bi 6s orbitals.12,38 The influences of the fluorination in the O and Bi (especially Bi) core levels, as evidenced by the XPS measurements, should be responsible for the lowering of the 20032

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valence band maximum (VBM) of the fluorinated Bi2O3 (RF = 0.2). The degradation of an organic pollutant over a photoirradiated semiconductor mainly refers to the behavior of the holes photoexcited on the valence band. The lowering of the valence band maximum (VBM) in the fluorinated Bi2O3 (RF = 0.2) is possibly more helpful for the degradation of organic pollutants with respect to the oxidation power of the holes on the valence band. 3.2. Photocatalytic Properties. To investigate the effects of the fluorination on the photocatalytic properties of the Bi2O3, the photocatalytic activities for the degradation of MO over the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values were evaluated under the visible light irradiation (λ > 420 nm) for 120 min, as presented in Figure 7.

still much higher than that of the pure Bi2O3 (RF = 0) and that the dependence of the activity on the RF values is also the same as before the normalization. Figure 8 presents the degradation of MO over the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2) in the presence of

Figure 8. Photocatalytic degradation of MO over (a) pure Bi2O3 (RF = 0) and (b) fluorinated Bi2O3 (RF = 0.2) in the absence and presence of different scavengers under the visible light irradiation.

different scavengers upon the visible light irradiation (λ > 420 nm). As observed in Figure 8, over the two samples, the degradation efficiencies of MO are significantly reduced with the addition of •OH scavenger TBA, and almost no MO degradation occurs when TEOA as an effective hole scavenger is added into the reaction solution. These results reveal that both •OH radicals and photogenerated holes as active species are involved in the degradation of MO over the two samples, whereas •OH radicals are major species in the photoreaction. Our previous studies found that the trace amount of the hydrogen peroxide (H2O2) could be produced over the irradiated Bi2O3 through the two-electron reduction process (reaction 1) in the presence of methanol as an electron donor.17 With the excitation of the two catalysts by visible light irradiation, thus, there are two possible production paths of • OH radicals in both systems. One is the reductive path of conduction band electron through reactions 1 and 2; the other is the oxidative path of valence band hole through reaction 3.

Figure 7. Photocatalytic degradation yield of MO over pure Bi2O3 (RF = 0) and fluorinated Bi2O3 with different RF values under visible light irradiation (λ > 420 nm) for 120 min. The inset is the normalized yield against RF values. Normalized yield is obtained by multiplying the MO degradation yield by the surface area ratio of pure Bi2O3 (RF = 0) to the corresponding catalyst.

The control experiment shows that the self-degradation of MO is negligible within the same irradiation time in the presence of no catalyst. The pure Bi2O3 (RF = 0) exhibits a very poor photocatalytic activity. This is attributed to the inability of its CB electrons to scavenge the surface oxygen molecules, resulting in a fast recombination of the photoexcited electron and hole. All fluorinated Bi2O3 with different RF values exhibit significantly higher activities than the pure Bi2O3 (RF = 0), indicating that the fluorination is an efficient treatment method for enhancing the photocatalysis of Bi2O3. Furthermore, the activity of the fluorinated Bi2O3 strongly depends on the RF value. With the increase of the RF value, the activity reaches an optimum for the fluorinated Bi2O3 with RF = 0.2, at which about 83% of MO was degraded under the irradiation for 2 h. After the optimum, the activity starts to drop when RF = 0.3. Based on the above XRD results, there exists minor impurity of BiOF in the fluorinated Bi2O3 (RF = 0.3). The conduction and valence band positions of BiOF are +1.97 and −1.73 vs NHE, respectively.39 Therefore, its presence diminishes the quantum yield and active sites of the fluorinated Bi2O3 (RF = 0.3), leading to a decrease in activity. It is well-known that the surface area of a catalyst could play an important role in its catalytic efficiency. As listed in Table 1, the surface area of the Bi2O3 is obviously increased after the fluorination. To ensure a fair evaluation of these photocatalysts’ activities, the photocatalytic degradation rates of MO are normalized in term of the surface areas, as depicted in the inset of Figure 7. It can be found that the normalized activities of all fluorinated Bi2O3 are

2eCB− + O2 + 2H+ → H 2O2

(1)

H 2O2 + eCB− → OH− + •OH

(2)

hVB+ + H 2O (or−OH) → •OH

(3)

The disappearance of the MO degradation rate in the presence of TEOA infers that, in the two systems, reaction 3 is almost sole source of •OH production, while the production of • OH through reaction 2 is minor or negligible. In other words, the photoexcited hole not only directly degrades MO as active species but also acts as a sole production source of •OH radical, as a result, intrinsically determining the final photocatalytic activity. The effective production yield of the photoexcited holes participating in both the degradation of MO and the reaction of •OH production strongly depends on the separation degree of the photoexcited electron−hole pairs. According to the results shown in Figure 7, thus, it could be concluded that an efficient separation of the photoexcited electron−hole pair is 20033

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successfully achieved over the fluorinated Bi2O3, which results in the higher photocatalytic activities. 3.3. ESR Measurements and Understanding of Enhanced Photocatalysis over Fluorinated Bi2O3. As evidenced by the characterization results above, two types of fluorine including the surface-adsorbed and lattice-substituted fluorine are induced onto or into the Bi2O3 after the fluorination. The surface-adsorbed fluorine, as formed by a simple ligand exchange between F− and surface hydroxyl group (−OH), is considered to play an important role in the separation of the photoexcited electron−hole pairs.19 Since the electronegativity of F− is much stronger than that of hydroxyl group (−OH),19,40 the surface-adsorbed F− acting as an electron-trapping site can hold the trapped electron more tightly than the surface hydroxyl group (−OH), as confirmed by the photocatalytic reduction reactivity of other fluorinated photocatalysts.20 In other words, there is a stronger electron storage capacity on the surface of the fluorinated Bi2O3 than on that of the pure Bi2O3. The electron storage on the surfaceadsorbed F− retards the photoexcited electron transfer to the surface O2, probably forming H2O2 through two-electron reduction process (reaction 1) in the present case,17 but the recombination of the photoexcited electron−hole pair is effectively inhibited.41 As a result, more photoexcited holes are allowed to react with surface H2O (or −OH) to form more hydroxyl radicals or directly oxidize the organic compounds, giving a rise to the photocatalytic efficiency over the fluorinated Bi2O3. This understanding of the enhanced photocatalysis over the fluorinated Bi2O3 is well supported by the ESR spectra analysis of the DMPO−•CH2OH spin adduct formed in the visible light irradiated methanol suspensions of the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2) in the absence of oxygen, shown in Figure 9. As reported earlier, in the present

evaluate the separation degree of the electron−hole pairs photoexcited over the pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2) in the presence of no any electron scavengers. It can be seen in Figure 9a,b that the characteristic ESR sextet peaks of a DMPO−•CH2OH spin adduct clearly appear over the fluorinated Bi2O3 (RF = 0.2) after visible light irradiation for 8 min, while no such signals are apparently observed on both samples in the dark, even on the pure Bi2O3 (RF = 0) within the same irradiation time. The generation of the apparent sextet peaks of DMPO−•CH2OH spin adducts well confirms that the electron storage capacity on the surface of the fluorinated Bi2O3 is much stronger than that of the pure Bi2O3, leading to an efficient separation of photoexcited electron−hole pairs. The ESR results also nicely parallels the results of the photocatalytic activities shown above. Meanwhile, the lowering of the valence band maximum (VBM) upon the substitution of F− for O2− in the Bi2O3 host after the fluorination, as evidenced by XRD and VB XPS measurements, equips the photoexcited hole with a more positive oxidation/reduction potential. From the viewpoint of the thermodynamics, such holes can more powerfully oxidize the organic compounds and react with the surface H2O (or −OH) to form hydroxyl radical. Therefore, the lowering of the VBM, we believe, also substantially contributes to the generation of the apparent sextet peaks of DMPO−•CH2OH spin adducts (Figure 9) and the enhanced photocatalysis over the fluorinated Bi2O3 (Figure 7). The above understanding of the enhanced photocatalysis over the fluorinated Bi2O3 can be schematically illustrated in Scheme 1. Scheme 1. Schematic Illustration of MO Photocatalytic Degradation over Fluorinated Bi2O3

Earlier investigations indicated the high degree of crystallinity facilitates the bulk diffusion of photogenerated charge carrier to the catalyst surface where photocatalytic reaction occurs.45 Thus, we cannot rule out the possibility that the slight improvement in the crystallinity of Bi2O3 after the fluorination (see Figure 1) might be favorable for the photocatalysis of the fluorinated Bi2O3. In summary, the fluorinated Bi2O3 has been synthesized through a precipitation method, followed by a solvethermal process. The fluorinated Bi2O3, especially with the atomic ratio of F to Bi at 0.2, exhibits a much higher photocatalytic activity than the pure Bi2O3 for the degradation of MO under the visible light irradiation. Both the surface-adsorbed and latticesubstituted fluorine in the fluorinated Bi2O3 play critical roles in the enhanced photocatalytic performance by inhibiting the recombination of the photoexcited electron−hole pair and increasing the oxidation power of the photoexcited hole. This work indicates that the fluorination is also a useful method for the development of the small band gap semiconductors with a deep valence band as efficient photocatalystic materials.

Figure 9. ESR signals of DMPO−•CH2OH spin adduct in (a) pure Bi2O3 (RF = 0) and (b) fluorinated Bi2O3 (RF = 0.2) methanol suspensions before and after visible light irradiation.

case, the photocatalytic oxidation of methanol by the photoexcited hole results in the formation of hydroxymethyl radicals through the reaction 4 only:42,43 CH3OH + hVB+ → •CH 2OH + H+

(4)

The conversion of methanol to hydroxymethyl radicals proceeds rapidly with a rate constant, k = 9.7 × 108 M−1 s−1.44 Herein, we attempted to detect the radical species to 20034

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ASSOCIATED CONTENT

S Supporting Information *

FESEM images of pure Bi2O3 (RF = 0) and fluorinated Bi2O3 (RF = 0.2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +8610-62514133; Fax +8610-62516444; e-mail jlin@ chem.ruc.edu.cn (J.L.). Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS The authors highly appreciate the financial support of this work from the National Natural Science Foundation of China (21273281), National Basic Research Program of China (973 Program, No. 2013CB632405), and the Fundamental Research Funds of Renmin University of China (92326121).

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